A Jacobus is an English

gold coin A gold coin is a coin that is made mostly or entirely of gold. Most gold coins minted since 1800 are 90–92% gold (22karat), while most of today's gold bullion coins are pure gold, such as the Britannia, Canadian Maple Leaf, and American Buf ...

of the reign of

James I James I may refer to: People *James I of Aragon (1208–1276) *James I of Sicily or James II of Aragon (1267–1327) *James I, Count of La Marche (1319–1362), Count of Ponthieu *James I, Count of Urgell (1321–1347) *James I of Cyprus (1334–13 ...

, worth 25

shillings The shilling is a historical coin, and the name of a unit of modern currencies formerly used in the United Kingdom, Australia, New Zealand, other British Commonwealth countries and Ireland, where they were generally equivalent to 12 pence or ...

. The name of the coin comes from the Latin inscription surrounding the King's head on the obverse of the coin, IACOBUS D G MAG BRIT FRA ET HI REX ("James, by the grace of God, of Britain, France and Ireland King").

Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a " natural philosopher"), widely recognised as one of the grea ...

refers to the coin in a letter to John Locke:

'' The Jacobus piece coin'd for 20 shillings is the : part of a pound Troy, and a Carolus 20s piece is of the same weight. But a broad Jacobus (as I find by weighing some of them) is the 38th part of a pound Troy.''Letter of Isaac Newton
dated September 19, 1698, to John Locke, concerning the weight and fineness of various coins.

These correspond to masses of 9.10 and 9.82

gram The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram. Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to th ...

s respectively, making the broad Jacobus slightly heavier.

References

Coins of England English gold coins {{coin-stub